Slotted cable localizer antenna

ABSTRACT

This invention relates to antennas for simultaneous transmission of sum and difference patterns as in a runway localizer for instrument landing of airplanes. A straight horizontal run of coaxial cable, placed centrally perpendicular to the extension of the runway centerline, is symmetrically fed from both ends by an RF bridge in sum and difference modes. The outer conductor of the cable is periodically interrupted by slots which permit currents to flow on the outside of the outer conductor. The antenna current distributions corresponding to the two modes are controlled by low-resistance shunts placed across the slots and by the relationship between the slots spacing and the internal guide-wavelength. To maintain the relationship over the 4 percent band of localizer frequencies, the guide-wavelength is adjusted always to the same value by filling the cable with the necessary quantity of a gas of relatively high-dielectric constant.

United States Patent Primary ExaminerEli Lieberman ABSTRACT: Thisinvention relates to antennas for simultaneous transmission of sum anddifference patterns as in a runway localizer for instrument landing ofairplanes. A straight horizontal run of coaxial cable, placed centrallyperpendicular to theextension of the runway centerline, is symmetricallyfed from both ends by an RF bridge in sum and difference modes. Theouter conductor of the cable is periodically interrupted by slots whichpermit currents to flow on the outside of the outer conductor. Theantenna current distributions corresponding to the two modes arecontrolled by low-resistance shunts placed across the slots and by therelationship between the slots spacing and the internalguide-wavelength. To maintain the relationship over the 4 percent bandof localizer frequencies, the guide-wavelength is adjusted always to thesame value by filling the cable with the necessary quantity of a gas ofrelatively high-dielectric constant.

T En HAY 4mm SHEET 2 [IF 2 L '1 6O 68-| "1 fi 62 FEED came 24 8 RUNWAYra I INVENTOR. 7/7/7777 rl SLOTTED CABLE LOCALIZER ANTENNA SUMMARY Anobject of the invention is to provide improvements with respect to sizeand performance over the slotted waveguide loealizers, now in use atsome airports, and which operate in the band l()8-l I2 m.c.p.s. inaccordance with the published description: C. B. Watts, Jr.,Simultaneous Radiation of Odd and Even Patterns by a Linear Array," IREProc., Vol. 40, pp. 1236-1239c, Oct. I952. A drawback of the slottedwaveguide localizer is the difficulty of retuning the antenna from oneoperating frequency to another especially in the case of the largerapertures. This is primarily due to the change with frequency of theguide-wavelength as it affects the relationship of the standing waves tothe slot positions. ln the present invention, a length of TEM coaxialtransmission line is employed instead of TE rectangular waveguide,resulting in much smaller cross-sectional dimensions. An advantageaccruing from this is the practical feasibility of filling the lineswith a high-dielectric gas, and using the gas pressure to control thevelocity of propagation and, hence, the guide-wavelength. With thefrequency limitation thus removed, the length of the antenna can besimply extended to almost any aperture that could be reasonably used inservice. With this technique, apertures of 40 wavelengths or more areconsidered practical.

LIST OF FIGURES FIG. I is a plan view of a rudimentary model of theslotted cable localizer antenna.

FIG. 2 is an approximate equivalent circuit ofthe antenna.

FIG. 3 is a set of graphs representing, for sum and difference modes,the desired relative slot excitations, with the corresponding radiatedfield pattern shapes.

FIG. 4 is a set of graphs illustrating, for sum and difference modes,the standing wavesaof line current, together with resultant slot voltagedistributions.

' FIG. 5 is a view of one section of the antenna. indicating a practicalconstruction which allows pressurization with a gas dielectric.

FIG. 6 is a cross-sectional view of one installation arrangement of theantenna over a ground surface, where the feed cable portion is made toserve as a reflector for the radiating slotted cable portion.

FIG. 7 is a plan view of a runway showing the antenna, with superimposedsum and difference polar patterns; not to scale.

DESCRIPTION A rudimentary form of the slotted cable localizer antenna isshown in FIG. I. Construction is made as nearly symmetrical as possible,end to end, about the centerline 2. For this reason. the view shows onlya little morethan half of the antenna. The coaxial transmission linebridge, or hybrid 4 may be of any suitable conventional type, the oneshown being known commonly as a rat-race," with phase-reversal providedby constructing one of the bridge arms 6 longer by a half wavelengththan the other three. It is a property of the bridge that opposite ports8 and I0 are isolated from each other if the loads presented at theother two ports 12 and 14 are equal to each other. This condition existsby virtue of the symmetry. Thus, signals can be applied independently orsimultaneously to the input ports 16 and 18 without interaction. Stubs20 and 22 are provided for separately impedance-matching each inputport.

From the bridge, signals proceed outward through left and right feedcable portions 24 and 26 to the ends of slotted cable portion 28. Herethe signals turfn inward toward the center, passing on the way the outerconductor slots 30. Each slot is shunted by a relatively heavy metalstrap or bar 32, which is securely clamped and conductively. connectedto the cable outer conductor by clamps '34.

Where the signals from left and right side meet in the cable at point 36on the centerline, considerations of symmetry dcmand that either avirtual short circuit or a virtual open circuit 75 exists, dependingupon which input port is considered.

FIG. 1 represents an approximate equivalent electrical circuit of theantenna. Corresponding parts are labeled with the same numerals as inFIG. I. The cable has a characteristic impedance, Z whose value is notimportant, as long as it is constant. It also has a velocity ofpropagation which, along with the operating frequency, determines theguide-wavelength, )t This is quite a critical figure, as the slotspacing, S, must differ from k by only a small length, 6. Theaccumulated difference, n8, over. one entire side of the antenna shouldhave a value preferably between 0.1 and 0.2 times the guide wavelength,where 2n equals the total number of slots in the antenna. FIG. 2indicates that at each cable slot, there is an impedance effectively inseries with the cable. Each impedance has two parts, the slot impedance38, and the shunt reactance 40. Impedance 38, due to the slot itself, isrelatively insensitive to position in the cable if the cable issufficiently long. It has resistive and reactive parts, the resistivepart being mainly due to radiation. Shunt reactance 40, due to bar 32and clamps 34, is a means of controlling the voltage drop acrossthe slotfor a given line current. As such, it has a maximum value in the centerof the antenna, tapering off towards the ends. Even the maximum value ofreactance 40,however, is made to be small in comparison with both slotimpedance 38 and line characteristic impedance Z,,. This means that thecurrents flowing in the cable are not greatly disturbed by the presenceof the slots.

Nevertheless, a small fraction of the total signal does escape througheach slot, causing currents to flow on the outside of the outerconductor, resulting in radiation. The polar pattern of the resultantradiation is computed by treating the problem as a long wire withmultiple excitations. One tends to think of a long wire as radiatingstrongly off the ends. This is because a long wire, as normally fed at asingle place, has a traveling wave-type current distribution withassociated end-fire radiation When, however, there are many excitationpoints,

phased properly for broadside addition, then the various traveling wavesproduced add destructively to a small resultant. The overall behavior ofthe long wire antenna then becomes almost identical with that of aeolinear broadside array of dipoles. The longer-the antenna, it wouldseem, the more nearly so is this. Even with as few as three or fourexcitation points, the resemblance is quite strong. See, for example,FIGS. 3 and 4 ofthe article by L. C. Shcn, The field pattern of a longantenna'with multiple excitations," IEEE Transactions on Antennas andPropagation, Vol. AP-l6, pp. 643- -b46 Nov. I968.

To a good approximation, if the number of slots is large, the fieldpattern may be computed, considering the slots as point sources, usingstandardarray theory. FIG. 3 illustrates the types of slot distributions(a) and (b), with corresponding field patterns (c) and (d), desired forthe localizer service. The sum and difference signals each produce theirown distinctive pattern known also by the names carrier" and sideband"pattern, respectively. The sum pattern FIG. 3 (c) is an even function ofazimuth angle, 0, an ideal shape in this application being that of theGaussian curve, l/exp((-)/k) while the difference pattern FIG. 3 (d) isan odd function of 9, having the shape of the Gaussian first derivative,9/exp(6/k)". The symbol, k, is simply a constant indicative of the beamwidth. These patterns are ideal in the sense that they are free of minorlobes and also that their ratio is a linear function of azimuth. In alarge aperture antenna, these patterns are formed to good apdiscussionof the formation of slot excitation distributions.

FIG. 4 (a) and FIG. 4 (h) are enlargements of portions of FIG. 3 (a) andFIG. 3 (h) respectively, intended to show the 3 spatial relation withthe standing waves'of line current associated with the sum anddifference modes, FIG. 4 (c) AND FIG. 4 (11), respectively. The sum modeis'so-named because currents arriving at centerline point 36 via the twopaths add venient, however, because it is simpler to deal with standingwaves of current rather than of voltage, since the slots are effectivelyin series rather than in shunt with the line.

Referring once more to' FIG. 4 (c), the sum mode standing waves near thecenterline have maxima which almost coincide with the slot positions.Away from the centcrline, however,

because the guide wavelength, )t is slightly longer than the slotspacing, s, the slot positions appear displaced from the current maxima,more and more, the farther they are from the centerline. If all of theslots had identical shunts, and if one neglected line loss due toradiation, and other losses, this displacement effect would result in aslot voltage distribution which would follow a cosine wave envelope,indicated by the curve 42, FIG. 4 (a). The slot shunts are, however, notidenti cal, but are made to have reactances that taper down therebyreducing the slot voltages, toward the ends of the antenna. Byadjustment of these reactances. then, the resultant sum mode slotvoltage envelope can be made to follow a desired shape, for example, theaforementioned Gaussian l/exp(x/a) indicated by curve 44, FIG. 4 (e). Ifall the shunt reactances are raised to increase the slot couplings tothe point where the effects of radiation loss are no longer negligible,it will be noticed that the standing wave minima are no longer deepnulls, but become more and more filled toward the ends; the standingwave ratio is lowered, with improved efficiency. While the antenna isalso operable in this manner. the detailed computation of performance isztgood deal more complicated than with the simplifying assumption ofloose slot-coupling. It is best then to use an iterative procedure witha digital computer to calculate line impedances as transformed througheach line section in succession, starting at the ccntcrline and workingout. i

Returning now to FIG. 4 (d), the difference mode standing waves near thecenterline have minima which almost coincide with the slot positions.Away from the centerline, however,

. the slot positions appear displaced from the current minima,

more and more, the farther they are from the centerline, If all theslots have identical shunts, this displacement effect would produce asine wave slot distribution, curve 46, FIG. 4 (b). The fact that theslot reactances taper toward the ends, however, modifies thedistribution shape to something like curve 48. If the shunt reactancetaper is assumed to modify the slot couplings in the same way for bothsum and difference modes,

' then the two distributions are distinguished from each other only bythe sine and cosine factors. Since the sine/cosine ratio is the tangentfunction, which is fairly linear for perhaps the first one-eighthwavelength, curve 48 is simply obtained by applying a linear multiplier,proportional to x, to the ordinates of curve 44. One thus reaches theconclusion, in the example cited, that curve 48 must approximate theshape of the Gaussian derivative, x/exp(x/a) I In order to keep theaccumulated difference n8 within the preferred limits, it is necessaryeither to build the antenna with just about the right slot spacingfs, orto have some means of adjusting the guide wavelength, A to suit. Aconvenient way to obtain limited adjustment of A, is to design theantenna to be pressurized, and to fill it with a high-dielectric gas.The A, is proportional to 1 VLC, where L and C are the line parameters,inductance and capacitance per unit length respectively. If the spacebetween inner and outer conductors of the line could be completelyfilled, the x, would then vary inversely as the square root of thedielectric constant of the gas. For squareroot localizer band, which issomewhat less than 4 percent, a gas with a dielectric constant of l.()8will suffice. The gas sulfur hexafluoride, SF,,, which has been usedcommercially as an electrical insulator, has the required dielectricconstant at a pressure of about 22 atmospheres. Other gases could, no

doubt, be-found that are suitable, such as some of the freon compounds,or other refrigerants, provided they are kept warm enough to avoidcondensation in the line.

In a practical type of construction in a size suitable for use at l l0m.c.p.s., the antenna is conveniently fabricated in sections about 8feet long, one slot in each, the sections being joined together on thesite by flange connections. FIG. 5 is a view of a section designed toallow pressurization with a gas dielectric. The drawing is ,not toscale, diameters being considerably exaggerated relative to the length,for clarity. The transmission line is of the rigid type, standard to theradio broadcasting industry, outer conductor pieces 50 and 52 beingbrazed to flanges 54 and 56 at each end of the section. The

inner conductor 58 is supported by dielectric pins 60 in the usual way.at intermediate points along the length, and anchored longitudinally bya dielectric bead 62, provided with ample holes 64 to allow free passageof gas from one section to the 'next.- An O-ring 66 provides a gas sealfor the flange con nection. The slot 68 may be constructed by sawing a/zinch piece out of a standard line section and cementing the pieces'into the insulating sleeve 70. As an alternative to the inductive slotshunts 32 of FIG. I, FIG. 4 shows a capacitive slot shunt, metal tube72, fitted tightly over insulating sleeve 70. The amount of energyescaping from the slot 68 is controlled by the length of tube 72 and bythe thickness of sleeve 70. If the length L of tube 72 is very shortcompared to the wavelength, the shunting effect is computed as a simplecapacitive reactance; if not, it is computed as a pair of transmissionline transformers of length L/Z each.

Two types of indicator may be provided to assure that the guidewavelength A has been properly adjusted. Indicator 74,

shown dotted, is a gas pressure gauge, mounted at any convenient pointon the antenna, indicative of the quantity of gas in the line. Indicator76, shown dotted, is a line current probe indicator, indicative of thecurrent flowing inside the line, and may be mounted at a place where afar-out standing wave minimum is supposed to fall. Such a place would becorresponding to point 78 in FIG. 4. Gas pressure is then simplyregulated to minimize the reading ofindicator 76.

FIG. 6 shows in cross section how the antenna may be mounted over agroundsurface using a number of wooden posts 79 with notched erossarms80. The feed-cable portion, which could be laid on or under the ground,is shown supported at the same height, h, as the slotted-cable portion,separated by a distance, d, where it can serve as a partial reflector.FIG. 7 is a plan view, not to scale, of arunway, with the antennalocated at one end. Curves 82 and 84 are indicative of the sum anddifference mode polar patterns produced.

It should be recognized that the ideas just described can be used inother physical arrangements. Other types of transmission line and otherslot-shunt arrangements will work just as well. The azimuth pattern andslot excitation distributions are not limited to those shownspecifically. Control of the guide wavelength can be obtained byintroduction of liquid or solid materials into the line, although not asconveniently as with gases. The antenna may also be used inother-orientations, such as vertically, to defirie a glide path.

Iclaim:

I. A slotted cable antenna comprising a centrally located hybrid withsum and difference input terminals, first and second transmission linesextending symmetrically outwards from the output terminals of saidhybrid, a third length of coaxial transmission line, connected at eachend respectively to the outer ends of the said first and secondtransmission lines, a plurality of gaps in the outer conductor of saidthird transmission line, radiation control means disposed across each ofsaid gaps, said m:ans presenting in each case an impedance, whereby mostof the line current is conveyed across 6 means comprising a metal tube,surrounding each of said gaps, but insulated therefrom.

4-. An antenna as in claim 3, with frequency tuning means comprising aquantity of high-dielectric gas filling the space between inner andouter conductors of said transmission lines.

1. A slotted cable antenna comprising a centrally located hybrid withsum and difference input terminals, first and second transmission linesextending symmetrically outwards from the output terminals of saidhybrid, a third length of coaxial transmission line connected at eachend respectively to the outer ends of the said first and secondtransmission lines, a plurality of gaps in the outer conductor of saidthird transmission line, radiation control means disposed across each ofsaid gaps, said means presenting in each case an impedance, whereby mostof the line current is conveyed across the gap, except for a portion ofsaid current which escapes to flow on the outside of said third coaxialline, thereby producing radiation.
 2. An antenna as in claim 1, withsaid radiation control means comprising a metallic shunt connectedconductively across each of said gaps.
 3. An antenna as in claim 1, withsaid radiation control means comprising a metal tube, surrounding eachof said gaps, but insulated therefrom.
 4. An antenna as in claim 3, withfrequency tuning means comprising a quantity of high-dielectric gasfilling the space between inner and outer conductors of saidtransmission lines.